1. Field of the Invention
The present invention relates to a power source device for high-frequency induction heating which is utilized for the heat treatment of a metallic object to-be-treated such as the surface quenching of a gear, a screw or the like.
2. Description of the Prior Art
Heretofore, for the surface quenching of a steel product, high-frequency induction heating has been practised for long by exploiting the phenomenon that a high-frequency current induced in the object to-be-treated by high-frequency electric power concentrates in the part of the object to-be-treated close to the outer surface thereof owing to the skin effect. However, in the induction heat treatment of the object to-be-treated having an uneven shape, for example, a gear or a screw, it is impossible with the high-frequency electric power of single frequency to homogeneously heat the convex part and concave part of the object to-be-treated and to attain a uniform depth of surface hardening over the whole object.
In recent years, therefore, a 2-frequency high-frequency heating method has been developed and performed as an induction heating method for the surface quenching of a gear, a screw or the like. First, a high-frequency current at a comparatively low frequency f.sub.1 is supplied to the heating coil of an induction heating apparatus so as to heat the concave part of the object to-be-treated, and when the temperature of the concave part has reached a predetermined temperature, the supply of the high-frequency current to the heating coil is once interrupted to stop the heating. Subsequently, a high-frequency current at a comparatively high frequency f.sub.2 is supplied to the heating coil so as to heat the convex part of the object to-be-treated, and when the surface temperature of the whole object to-be-treated has reached a desired quenching value, the supply of the high-frequency current to the heating coil is stopped, and the object to-be-treated is rapidly cooled. Thus, the object is quenched.
In this regard, for the purpose of performing a quenching treatment of high quality, it is necessary to generate a desired uniform surface temperature over the whole object to-be-treated. In the 2-frequency high-frequency heating method of the prior art stated above, the timing of frequency switching for generating the desired uniform surface temperature is very important, and frequency switching conditions for the appropriate heat treatment need to be established beforehand by repeatedly conducting preparatory tests. Therefore, the productivity of the method is low. Moreover, the method involves the problem concerning control that, in a case where the timing of the frequency switching has been missed, no adjustment is possible. A further disadvantage is that two high-frequency power sources of the different frequencies are required for supplying the heating current to the heating coil.
Therefore, an object of the present invention is to provide a power source device for high-frequency induction heating which can eliminate the above mentioned disadvantages of the conventional methods.